The field of the present invention generally is integrated circuit packaging.
A PGA package has pins protruding downward from its body and is inserted into a socket that contains female type openings that correspond to the pins on the PGA. The socket is soldered to a circuit board either directly onto the surface or in plated through holes.
The PQFP has output leads extending downward and outward from the body and is designed to be soldered directly onto the surface of a board, i.e., surface mounted. The leads on the PQFP are disposed in rows around the periphery of the package. The LGA package has leads that are almost flush with the bottom of the integrated circuit package, and like the PQFP, is soldered directly onto the surface of a circuit board. LGA packaged integrated circuits generally have their leads disposed in two different fashions. A first type of LGA package disposes its leads in a matrix that covers the entire bottom surface of the package. A second type of LGA disposes its leads around the periphery of a package, such that the bottom of the package has an area that is surrounded by but does not contain leads.
When comparing the PQFP, LGA and PGA packages, it is seen that use of a PGA package is often more desirable than either PQFPs or LGAs because board insertion requires no soldering. Additionally, the use of PGA packages allows for simple component removal and upgrading. Because of these advantages, circuit boards are often designed to accept PGA packages.
In a modern manufacturing environment, parts are often delivered to the factory as close to the time they are needed as possible, so-called "just in time delivery." Since PQFP and LGA packages cannot be substituted for PGA packages, should a PGA packaged part be unavailable on a particular day, an entire manufacturing facility could be shut down. This situation has created the need for a product that allows flat pack and LGA packaged integrated circuits to be inserted into PGA package sockets.
An add-on card that allows PQFP or LGA packaged integrated circuits to be inserted into PGA package sockets allows for more convenient manufacturing. By pro-soldering quad flat-pack packages to the add-on board, the user can produce large quantities of integrated circuits that can be inserted into PGA package sockets. Since the PQFP-to-add-on card or LGA-to-add-on card assembly is manufactured prior to reaching the factory floor, the PQFP or LGA packaged integrated circuit can be inserted into the PGA package sockets at any stage during assembly without tedious soldering procedures. This saves circuit board production time and facilitates "just in time delivery." Therefore, an inexpensive add-on card that accomplishes this conversion is desirable.
An inexpensive and durable add-on card improves the quality of products containing PQFP and LGA packaged integrated circuits. Higher quality is accomplished because such an add-on card would allow for individual testing of the integrated circuit prior to insertion onto the board. This testing checks the functionality of the integrated circuit as well as the integrity of the package-to-add-on card soldering. Without the use of an add-on card, the entire board must be tested in order to check package-to-board solder integrity.
The use of an add-on card also leads to cost savings opportunities. Because PQFP packaged integrated circuits use less expensive materials and manufacturing processes, they cost substantially less than PGA packaged integrated circuits. In fact, the combined cost of the PQFP packaged integrated circuit, an add-on card, and assembly cost is still lower than a PGA packaged integrated circuit, thereby lowering the cost of the product containing the PQFP.
The use of an add-on card provides many other advantages as well. The use of an add-on card allows for a reduction in the amount of part inventory a factory keeps stocked. This helps to achieve a "just in time" manufacturing scheme. In a just in time delivery scheme, as discussed above, parts are delivered to the factory at as close a time to their need as possible. This has been found to reduce cost and improve quality. Performing a just in time delivery scheme, however, is difficult. Any problems in execution can lead to manufacturing costs that exceed the cost of not using the system. Since maintaining large inventories of integrated circuits and computer boards is a large expense, it is desirable to have a low inventory. By using an add-on card, the integrated circuits can be pre-soldered onto it, stored on shelves, and installed on computer boards at shipping time. Integrated circuit inventory is kept low because it is easier to solder PQFP packaged integrated circuits onto add-on cards than to solder them onto computer boards.
Another advantage of the use of an add-on card is the ability to easily upgrade components. When newer or better performing integrated circuits become available, it is desirable and often necessary to replace the older ones. It is also desirable to be able to perform this change at either the factory or the user. Both flat packs and LGAs are soldered directly to circuit boards. Therefore, neither can be removed from a board without a high probability of component or board damage. By using an add-on card, the PQFP-to-add-on card or LGA-to-add-on card assemblies become modules that can easily be inserted or removed from circuit boards, thereby making component upgrade feasible.
There are three different basic types of prior art add-on cards. The first prior art add-on card uses a fiberglass board. The leads of the add-on card are either punched or screwed into the board. On the side of the board opposite the pins are bonding pads. The pins of the add-on card are placed in electrical connection with the bonding pads by routing vias from either the pin side or bonding side of the board to the pin. The via is electrically connected to the pin at either the point where the pin emerges to the bonding side of the board or where the pin enters the hole on the pin side of the board.
This type of board suffers from shortcomings. Firstly, because the vias must be disposed throughout the surface of the board, shielding devices for shielding electromagnetic interference cannot be used on either side of the board. Secondly, the attachment and alignment of the flat pack on the card is cumbersome and cost intensive.
A second prior art add-on card uses a two layer circuit board. The first layer holds the pins and the second layer holds the bonding pads. The pin layer has holes formed all the way through. Pins are inserted within the holes and soldered. There is also an array of connection holes formed in the interior surface (the side that is attached to the bonding pad layer). The connection holes are electrical conductors formed therein. The pins are placed in electrical connection with the bonding pads by routing a via on either the bottom of the pin layer or top of the pin layer from the pin to one of the electrically conductive connection holes.
The bonding pad layer is attached to the pin layer by having a matrix of pin points disposed on its interior surface (the side that is attached to the pin layer) placed in the electrically conductive connection holes in the pin layer. The pin points are routed to the bonding pads by vias disposed on the top surface of the bonding pad layer. The bonding pad layer has electromagnetic shielding on both its interior surface and bonding pad surface. However, no shielding is placed on the pin layer. This can cause interference with the signals in the vias of the pin layer. Additionally, as with the first type, the attachment and alignment of the flat pack is cumbersome and cost intensive.
A third prior art add-on card uses a single layer circuit board. The pin surface of the circuit board has pin pads formed thereon that are routed by a system of vias on both sides of the board to the mounting pads. In order to fasten the pins to the board, a plastic brace must be formed around the pins. The plastic brace holds the pins in an array that allows insertion into a PGA socket. Solder paste is placed on the pin side of the board in an array that matches the footprint of the pins. The pins, held in proper form by the plastic brace, is then placed onto the pin side of the board such that the tops of the pins are placed in the solder paste. The plastic brace used in this add-on card provides disadvantages of increased material cost and increased weight. Additionally, the pins which were held by the plastic brace tend to separate from the board after the PQFP is installed on the board and placed into the reflow oven. This separation occurs because the plastic brace and the fiberglass board have different thermal coefficients of expansion. This can cause quality problems. Finally, because the board has vias routed on both sides of the board, a shielding plate cannot be used. This can increase electromagnetic interference problems.